Like alioops said, it's only a problem when it becomes a problem for you. With DD I lay on my back all the way through my pregnancy with no effects..this pregnancy if I lay on my back for more than a few minutes I feel lightheaded. It's different in everyone, I think they just recommend against it so that those women that will be effected are aware of it. You can't help what you do in your sleep either! I would only worry if you get dizzy or your feet get numb or you feel weird. Sleep peacefully otherwise (whilst you still can!).